The present invention relates to motor drive technology, and more particularly, to a motor drive technology of a pulse width modulation (PWM) system.
As PWM drive systems for a brushless motor, a triangular wave slicing system and a peak current detecting system are known. In the triangular wave slicing system, a coil current is made to flow through a detection resistance, and the difference between a voltage generated at the detection resistance and a torque command voltage is output as a slice level by an error amplifier. A triangular wave having a constant period is sliced with the slice level, to determine the time period (ON period) during which the current flows to the coil. In the peak current detecting system, which uses no error amplifier, supply of a current to a coil is halted when the voltage generated at the current detection resistance, through which the coil current flows, reaches the torque command voltage, and a regenerative current mode is started.
FIG. 18 is a block diagram of a conventional motor driver of the peak current detecting method. Referring to FIG. 18, Hall sensors 21A, 21B and 21C detect the position of a rotor of a motor 10 and output the detection results to a position detection circuit 22 as Hall sensor outputs S11, S12 and S13, respectively. The position detection circuit 22 determines position signals S21, S22 and S23 based on the Hall sensor outputs S11, S12 and S13, respectively, and outputs the signals to a phase switch circuit 93. The position signals S21, S22 and S23 are signals obtained by shifting the phase of the Hall sensor outputs S11, S12 and S13 by 30°.
The phase switch circuit 93 determines the phases of currents to pass according to the position signals S21, S22 and S23. For easy measurement of the phase currents, the phase switch circuit 93 blocks flow of one of three phase currents. A Logic control circuit 95, set upon receipt of a reference pulse PI, controls supply of currents to the motor 10 by changing the level of signals output to the phase switch circuit 93. The reference pulse PI is a periodical pulse.
FIG. 19 is a graph showing changes with time of phase currents for the motor driven by the motor driver of FIG. 18. In FIG. 19, phase currents I1, I2 and I3 in U, V and W phases, respectively, are shown, and currents flowing from drive transistors 1 to 6 toward the motor 10 are considered positive. As is found from FIG. 19, there is always one phase current that becomes zero, and thus there occurs sharp change of any of the phase currents every electrical angle of 60°.
Assume that the logic control circuit 95 has been set with the reference pulse PI. The phase switch circuit 93 turns ON only the W-phase upper side drive transistor 5 and the U-phase lower side drive transistor 2, for example. In this state, a current flows to a current detection resistance 7 via a W-phase coil 13 and a U-phase coil 11. The magnitude of this current can therefore be detected as the voltage generated at the current detection resistance 7. Since this current flows through the inductive coils, the current gradually increases after the conduction of the drive transistors 2 and 5.
With increase of the current, the voltage generated at the current detection resistance 7 increases, and when it reaches a torque command voltage TI, the level of the output of a comparator 96 changes, causing the logic control circuit 95 to be reset. The reset logic control circuit 95 reverses the level of a signal output to the phase switch circuit 93. On receipt of this signal, the phase switch circuit 93 turns OFF the drive transistor 2.
The time period from the setting of the logic control circuit 95 until the reset thereof corresponds to the “on” period of switching operation. After the reset of the logic control circuit 95, the current flowing through the coils 11 and 13 still attempts to continue the flow, and this causes a regenerative current to flow through a diode 1D existing between the source and drain of the drive transistor 1. Since the regenerative current does not pass through the current detection resistance 7, the voltage generated at the current detection resistance 7 is zero during the flow of the regenerative current.
The regenerative current gradually decreases. However, upon receipt of the reference pulse PI, the logic control circuit 95 is set again, and the phase switch circuit 93 turns ON the drive transistor 2. This operation is repeated until the phase switch circuit 93 switches the phases of currents to pass. In this way, as a result of the alternate flow of the drive current flowing when the logic control circuit 95 is set and the regenerative current flowing when the logic control circuit 95 is reset, a phase current roughly corresponding to the torque command voltage TI is allowed to flow through a predetermined coil.
FIG. 20 is a graph showing the current detection resistance voltage (motor current detection signal) MC and the V-phase and W-phase currents I2 and I3 at and around time t=tz in FIG. 19, obtained by enlarging the time axis. In FIG. 20, a period T91 is a time period during which a drive current of the U-phase and V-phase currents flows. This drive current flows through the current detection resistance 7. A period T92 is a time period during which the U-phase and V-phase currents flow as a regenerative current. A period T93 is a time period during which a drive current of the U-phase and W-phase currents flows. This drive current flows through the current detection resistance 7. A period T94 is a time period during which the U-phase and W-phase currents flow as a regenerative current.
The conventional motor driver shown in FIG. 18 has the following problem. The phase currents sharply change as shown in FIG. 19. For this reason, when the phase currents are switched, vibration of the motor and generation of electromagnetic noise tend to occur.
To avoid the above problem, the phase currents may be controlled not to change sharply. However, to detect and control a plurality of phase currents, it is necessary to provide current detection resistances in the same number as the number of phases. It is difficult to incorporate the current detection resistances in an integrated circuit. Therefore, as the number of the current detection resistances is greater, the scale of the device is larger and the cost is higher.
In addition, the properties of resistances generally have variations. Therefore, in the case of using current detection resistances for the respective phases, the current detection properties vary every phase. For example, when two phase currents are actually the same in magnitude, the magnitudes of the detected currents may sometimes be different from each other.